This invention relates to the removal of hydrogen sulfide from gases, and more particularly to a process for removing hydrogen sulfide from hydrogenated Claus process tail gas.
With the increasing concern over the pollution of the atmosphere, great demands have been made on industry to produce pollution-free products and to do so in a pollution-free manner. One area of particular concern has been the release of sulfur and its compounds into the atmosphere during the refining of petroleum, the sweetening of sour natural gas, the processing of ore, the destructive distillation of coal, and during other processes which produce sulfur-containing gases.
A process widely employed for the desulfurization of hydrogen sulfide containing gases is the Claus process. The Claus process is about 90 to 97% effective in converting hydrogen sulfide to elemental sulfur and while substantial amounts of sulfur are recovered, thus preventing its release into the atmosphere, in many instances it is necessary to further treat the Claus process tail gas to make it acceptable for discharge into the atmosphere. Several processes are available for treating such tail gases including, for example, the Beavon sulfur removal process described in U.S. Pat. No. 3,752,877. This process involves the hydrogenation of the Claus process tail gas to convert the various sulfur compounds to hydrogen sulfide, and then employing a gas/liquid contact process to remove the remaining hydrogen sulfide. Representative of such gas/liquid contact processes is the Stretford process, a well-known process for converting hydrogen sulfide to elemental sulfur which is described in U.S. Pat. Nos. 2,996,439, 3,035,889, and 3,097,926. The Stretford process involves the contacting of the hydrogen-sulfide containing gas stream with an alkaline washing solution containing an alkali to maintain the pH, anthraquinone disulfonate (ADA), and a water-soluble metal vanadate. The hydrogen sulfide is oxidized to elemental sulfur which is subsequently separated and recovered from the washing solution. The washing solution is subsequently regenerated with oxygen and recycled to the gas contacting step. Other processes are also available which involve contacting the gas with an aqueous alkaline washing solution containing a regenerable oxidizing agent.
A major problem with these types of processes resides in the formation of salts in the washing solution, particularly thiosulfate salts. The formation of these salts results in a consumption of the alkali and, unless removed, these salts can build to a level where serious corrosion and process fouling problems will occur. Conventionally, a portion of the washing solution is bled off periodically for disposal while fresh makeup solution is introduced into the process. The removed solution represents a disposal problem and, in addition, represents a substantial waste of active components, particularly of the ADA and vanadium which are expensive.